When only a C1 – C2 arthrodesis is done, we prefer to use a C2 pedicle screw over a C2 pars screw. This is because the pars screw starting hole is more distal and therefore requires more muscle dissection. But, if a C2 pedicle screw is not possible, then we place a C2 pars screw.
2. Preoperative evaluation
A preoperative CT image must be evaluated to determine if this screw is possible. Some patients have a pedicle that is too small to accept such a screw.
3. Screw entry point
The pedicle screw starts more cranially than the pars screw and it is directed medially.
To find the starting point for the pedicle screw, draw a line along the cranial leading edge of the C2 lamina (1).
Then, draw a line along the midpoint of the pars mediolaterally (2) (see illustration).
Burr a starting-hole 2 mm lateral to this intersection.
Use either a pedicle probe or a handheld drill to drill the hole for the pedicle screw.
The direction of the drill is approximately 30° – 45° medial and craniocaudally angled to the bottom half of the tubercle of C1.
5. Screw insertion
Tap, measure the length, and place the screw. Typically a unicortical screw that is longer than 20 mm is adequate.